1. Field of the Invention
This invention relates to the field of electronic circuits associated with operational amplifiers (op-amps) and in particular to low-voltage versions of these circuits suitable for use in various pipelined analog-to-digital converter and/or other portable applications.
2. Brief Description of the Known Art
FIG. 1 shows a schematic for the well known xe2x80x98telescopic cascodexe2x80x99 amplifier. A bandwidth of 500 MHz and a dc gain of 60 dB is achievable with this type circuit. This differential op-amp is comprised of cascoded n-channel input transistors 1, 3 and 2, 4 at the Vip and Vin inputs, respectively, along with the long tail current source transistor 5. P-channel transistors 6-9 act as an active load. The common mode adjust circuitry 10 is used to compensate for a mismatch between tail current and the bias current by providing a common mode feedback (CMFB) signal to the gates of transistors 8 and 9. The differential outputs VOp and VOn for the circuit are taken from the drains of n-channel transistors 3 and 4, respectively. Because this amplifier is known to have a minimum number of non-dominant poles, the circuit has excellent bandwidth properties. This is primarily due to the following:
1) the p-channel transistors 6-9 in the circuit do not carry any signal current. Thus, all capacitances associated with p-channel transistors 8 and 9, as well as the gate-source and source-substrate capacitances of p-channel transistors 6 and 7 do not contribute any poles;
2) since the main non-dominant poles are contributed by the n-channel transistors, which can be kept small in size by tailoring their W/L layout for a given transconductance, the resulting poles will generally be at higher frequencies;
3) the large capacitance often associated with a folded cascode amplifier, where three transistors have to xe2x80x98meetxe2x80x99 at the folding node, is not present in this telescopic cascode amplifier.
The main problem with this conventional cascode operational amplifier circuit is that of low voltage headroom. Because so many transistors are stacked, it is difficult to realize this circuit when working at voltages as low as 2.5 volts or less.
A number of patents may be of interest in relation to the technology discussed above and in this patent, including but not limited to the following: U.S. Pat. No. 5,798,673 Low voltage operational amplifier bias current and method. U.S. Pat. No. 5,734,296 Low voltage operational amplifier input stage and method.
A new differential, low voltage, broadband operational amplifier which eliminates the long tail current source from the amplifier, thereby relieving the low voltage headroom requirements, is disclosed. This advantage will allow the amplifier to operate from a power supply of less than 2.5 volts.
The circuitry of this invention compensates for the significant changes in drain current in the input transistors of the amplifier by monitoring the current in a common mode feedback signal. When the value of this current exceeds a specified range around the nominal amplifier bias current, an appropriate correction is made to the common mode input voltage. Using this circuitry which can be implemented with minimal chip area and negligible power consumption, an operational amplifier is built without a long tail current source that operates at voltages of 2.5 volts or less.
Two implementations for the control loop for common mode input voltage are discussed, although other implementations are possible. Simulated results for both a conventional amplifier and the operational amplifier of this invention, both running from a power supply of less than 2.5 volts, are compared.